杏吧原创

As Arctic sea ice dwindles, a gull that depends on it seems to be disappearing from the ocean around Greenland.

range throughout the Arctic, spending their lives on or near pack ice in the far north. They survive on whatever food they can turn up: fish and invertebrates, the faeces of other animals, scavenged bits of seals killed by polar bears.

But the Canadian population of nesting Ivory Gulls was known to be in steep decline. Their numbers in the early 2000s were than in the 1980s.

鈥淲e鈥檝e noticed big changes in where the colonies are, the numbers of birds seen,鈥� says at Pomona College in California. It鈥檚 difficult to visit every potential nesting site as some are remote, 鈥渂ut in the places we鈥檙e going, there鈥檚 a lot fewer,鈥� she says.

To find out whether the gulls living near the Greenland Sea are also declining, of the Royal Belgian Institute for Natural Sciences in Brussels turned to data he collected aboard an icebreaker travelling between Greenland and Svalbard between 1988 and 2014.

Joiris found about six times more ivory gulls each year before 2007 than he found after that year. But it鈥檚 not clear what caused that change, says at the Canadian Wildlife Service. The birds may have died, or perhaps they simply moved on to a different location, he says.

鈥淭here seems to be population decline in some locations, and stability in others,鈥� he notes, referring to the population throughout the Arctic.

Indispensable ice

Arctic seabirds like the ivory gull rely on sea ice, says at McGill University in Montreal. It influences every aspect of their lives, including how they forage. 鈥淭hey鈥檙e flying out in the middle of the ocean, and it鈥檚 hard to find a school of fish,鈥� Elliott says.

The ice itself can be a sort of fish-finder, though: places where fresh water ice melts into the ocean are rich in algae and invertebrates, which in turn attracts fish.

Of all the Arctic seabirds, ivory gulls are most closely associated with sea ice, Karnovsky says. That makes them particularly vulnerable to its loss. If ice doesn鈥檛 form near where they鈥檙e breeding, for example, they might not be able to feed their chicks.

鈥淭hey have to be able to commute to a dependable supermarket, and often times that鈥檚 the sea ice,鈥� she says.

While shipboard surveys like Joiris鈥檚 are useful, Karnovsky says, other types of studies are needed to tease out what caused the drop in ivory gulls near Greenland.

Tracking survival and migration patterns could help researchers figure out what鈥檚 going on before it鈥檚 too late, Elliott says. 鈥淚f that Arctic ecosystem disappears 鈥� if the ice disappears 鈥� they鈥檙e not going to be able to survive.鈥�